Cunobelin

In the first century A.D., Cunobelin was the first (self-declared) king of the Britons(rex Britannorum). He united a good number of the early Celtic tribes (well, again, declared himself as the king of the early Celtic tribes). He originally welcomed Roman traders, but as Roman began it's conquest to conqueror Britian, he rallied his tribes against the forces Roman Emperor Aulus Plautius.

He died sometime in 40-41 A.D., a few years before Rome's conquest of Britain was complete.